The
Indian Ocean is the third largest ocean and includes four distinct sub
regions:
Western Indian Ocean, Central Indian Ocean,
Middle Eastern Seas and Southeast Asia It is closed to the
north, and a large proportion of its waters are tropical or near
tropical. Unlike the Atlantic it is largely bounded by relatively arid
countries and does not receive particularly high inputs of freshwater or
terrestrial sediments. The great exception to this is the Bay of Bengal
in the northeast , which is fed by massive riverine discharge by a
number of rivers, leading to conditions of high sediments and low,
fluctuating salinities -conditions inimical to coral reef development.
To the northwest are the enclosed sea areas of the Red Sea and the
Arabian Gulf, with very different tectonic histories, but both occurring
in highly arid regions with little terrestrial runoff. The coast
of East Africa is also relatively dry. Continental shelf areas are
generally narrow, although there are a few near shore island groups
which are important for coral reef development. There are also several
oceanic island groups, notably in the west and central parts of this
ocean.
There are large areas of coral reef right
across this region, making up nearly twenty percent of the world total.
Fringing reefs predominate along much of the Red Sea, particularly
northern and central parts. Further south in the Red Sea and Arabian
Gulf coastal sediments and high salinities restrict fringing reef
development, though there are extensive offshore patch reefs. Cool
upwellings limit the development of true reefs along parts of southern
Arabia and Pakistan. Further south there are fringing communities on the
coasts of East Africa, and particularly along the shores of the
continental islands. Some of the best developed reef structures occur in
isolated oceanic locations. There are numerous atolls and platform
structures in the west and central regions of the oceans, and the
Maldives and Chagos Archipelago include the largest atoll structures in
the world. The continental coastlines of India and Sri Lanka have very
limited reef development as there are various adverse conditions,
including high turbidity, fluctuating salinity and high wave energy.
There are important though little known reefs around the Andaman and
Nicobar islands and to the north of Sumatra. Australia also has
significant reef communities, including extensive fringing reefs,
offshore platforms and barrier structures and high latitude communities.
Species diversity is high across the
region, following a narrow band of high diversity in the Central Indian
Ocean and forming two distinctive sub-centers of diversity in the
Western Indian Ocean and the Red Sea. Elsewhere there is greatly reduced
diversity, notably in the Arabian Gulf and along the shores of mainland
India. Despite their high latitude there is relatively little diminution
of diversity in the reefs of the northern Red Sea. By contrast there are
latitudinal declines in species numbers, and as a consequence in the
development of reefs themselves, in both southern Africa and Western
Australia.
Wide
areas of this region were affected by the 1997-98 El Nino Southern
Oscillation event, and in 1998 warm waters swept across wide areas of
the Indian Ocean, leading to bleaching and massive levels of coral
mortality on reefs from Western Australia to the shores of East Africa.
Direct human pressures on coral reefs in the Indian Ocean are highly
varied. The Arabian Gulf contains the largest concentrations of oil
reserves in the world, and there is chronic oil pollution in this sea,
exacerbated by occasional massive oil spills. Tanker traffic also
carries the threat of oil pollution to other areas, notably the narrow
straits at the mouth of the Arabian Gulf, and at the northern and
southern ends of the Red Sea. Coastal development is sporadic -there are
vast areas of the Arabian coastline with little development, but in
others, such as around the major ports and some of the tourist areas of
East Africa, coastal development is having a direct impact on reefs.
Tourism is sporadic, but is critical to the economies of Egypt, Kenya,
Tanzania and the islands of the Indian Ocean.
Southeast Asia
is one of the most important areas in the world for coral reefs. Over
thirty percent of the world's reefs are found in this complex region
which straddles the waters between the Indian and Pacific Oceans. The
region includes the continental coastlines of Myanmar and Thailand,
Malaysia, Cambodia, Vietnam and China. The most extensive coastlines,
however, follow the great complex of islands which are dominated by the
Philippines and Indonesia. Indonesia alone has over 50,000 square
kilometers of coral reefs, nearly eighteen percent of the world total.
Japan lies on the edge of this region, and also has a considerable area
of reefs surrounding island chains which follow natural clines of
diminishing species diversity. The coastlines define a large number of
partially enclosed seas. While the waters around the continental shores
are generally shallow, deep oceanic waters come in close contact with
offshore islands in many areas.
Fringing reefs predominate, although there
are also extensive barrier reef systems, and a number of atolls and
near-atolls. This region is the great center of coral reef biodiversity,
and there are more species here, in almost all animal groups, than
anywhere else. To some degree this diversity is encouraged and
maintained by the complexity of the coastline and the great range of
habitats found in the region, but its ultimate origins can be traced
back over geological timescales. While extinctions were occurring in
other regions it would appear that species were able to survive in this
region, and even to diversify as sea levels fluctuated and areas became
isolated and then reconnected with one another.
Unfortunately this region is also the most
threatened and disturbed by human activities. Some eighty-two percent of
the region's reefs were considered to be threatened by human activities
in the
1998
Reefs at Risk Study. Most of these threats are linked to the
rapidly growing economies and populations in this region. These are
driving massive changes in the landscape, with forest clearance and
agricultural intensification leading to increased sedimentation and
pollution from agricultural chemicals. Massive urban expansion has also
led to enormous pollution problems close to urban centers. Fishing
pressures are ubiquitous, from chronic over fishing for local
consumption and the highest rate of blast fishing (fishing with
explosives) in the world, to target species over fishing even in many of
the remotest parts of the region.
Extracted from The World Atlas of Coral Reefs, by Mark D. Spalding,
Corinna Ravilious and Edmund P. Green, published by the
University of California Press .
For more complete and in-depth coverage of the topics presented in
this webpage, I recommend highly purchasing a copy of this
beautifully illustrated book. Just click on the University of
California Press link above to do so.
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